DESCRIPTION OF THE EXPEDITION 7 



To this Report is appended Capt. Somerville's " Observations for Terrestrial 

 Magnetism." The observations made at Funafuti appeared to us of considerable interest, 

 but we were unable to carry out similar work in any of the atolls we visited, only 

 indeed occasionally swinging the ship and getting a day ashore. It is generally believed 

 of course that there is no connection between the variation and the problems we were 

 investigating, but it is clearer day by day that all the sciences are becoming more and 

 more interrelated and the field worker must leave no stones unturned. The available 

 data relating to variation are very small. All that can be said at present is that the 

 ocean depths are relatively permanent as compared with the cyclical, annual and even 

 hourly changes in the variation. One may, we think, be quite certain that so enormous 

 a natural cause as terrestrial magnetism must have some equally large effect — it itself 

 being the result of a still more enormous cause — on the life that inhabits its field of 

 influence, while its effect on earth-movements must have been very great. The peculiar 

 behaviour of the curves of equal magnetic variation to the north of Madagascar, first 

 worked out by Capt. Somerville, is especially to be observed (fig. 1, p. 10), the line 

 from Seychelles to Madagascar being supposed to be that of an ancient land, com- 

 paratively recently submerged. 



B. OBSERVATIONS FOR TERRESTRIAL MAGNETISM. 



By Commander Boyle T. Somerville, Royal Navy, 

 commanding H.M.S. " Sealark." 



Among the scientific observations of a practically useful character, made whilst 

 H.M.S. "Sealark" was conveying the Percy Sladen Trust Expedition through part of 

 the Indian Ocean in 1905, were those for Terrestrial Magnetism. 



These may be conveniently divided into two headings ; namely : 



(1) Those taken afloat with the ship's Standard Compass : and 



(2) Those taken ashore, with Absolute Instruments. 



(1) Observations Afloat. 



The actual method employed was by swinging the ship both ways and making 

 observations with the ship's head on the points N., N.E., E., S.E., S., S.W., W., and 

 N.W., the eight-point swing as it is termed. It is necessary to keep the ship's head 

 steadily on the point at which the sun's bearing is to be observed for a period of about 

 five minutes ; for it is found that a compass needle does not immediately acquire all the 

 effect of local attraction, but has a tendency to lag behind, a peculiarity known as 

 hysterisis. In thirty double swings a difference was found on every occasion between the 

 variation resulting from the swing to starboard and that to port. This difference was 

 invariably in the same direction, the average being 20', the variation from the swing 

 to starboard being the greater in Westerly Variation. The eight-point swing, both 

 ways, takes at least one hour and forty minutes. For the best results a smooth sea 



